In continuously extruded blow molding processes, a hollow tube of melted polymer, or “parison”, is typically extruded through an annular die and into a moving mold. As the parison enters the moving mold, it is expanded with pressurized air or gas to force it into the shape of the mold. In some cases, a vacuum is also applied to the cavity within the mold to draw the parison against the interior walls of the mold. As the molten parison takes the hollow shape of the mold, and travels with the mold away from the hot extruder, air or gas within the hollow center of the molded polymer cools. As the air cools and becomes denser, a partial vacuum may form in the hollow center of the molded polymer. In some instances, the vacuum formed in the hollow center of the molded polymer may cause the molded plastic shape of the polymer to exhibit inner wall roughness, or even to collapse, unless a venting hole is formed to allow air to enter the hollow center before the molded plastic fully sets.
When discrete products, such as plastic containers, are intermittently blow molded, these products may be already vented and/or released from their respective molds and punctured within sufficient time to prevent a cooling vacuum from causing product deformation. Likewise, if a relatively small diameter and/or short corrugated pipe is continuously-extruded and blow molded, for example, a venting hole may be punched into the pipe as soon as the pipe releases from a mold but before the pipe becomes susceptible to cooling-induced deformation.
It has been found, however, that some products may be too large, or continuously-extruded too slowly, to avoid deformation if only vented after being released from their respective molds. For example, in the manufacture of large diameter corrugated pipe, such as that disclosed in U.S. patent application Ser. Nos. 11/078,323 and 12/019,738 to Goddard et al., it may not be sufficient to punch venting holes into the pipe only after the corrugation molds have released the pipe. Such large diameter corrugated pipe may be disposed inside a corrugator or vacuum chamber for a period of time (i.e., “residence time”) so long that the pipe may be subjected to cooling-induced deformation. Blow molding of dual wall corrugated pipe may be especially susceptible to cooling-induced deformation because a smooth inner wall may seal off an annular area associated with each corrugation of the pipe.
Accordingly, there is a need for an in-mold punch apparatus and related methods for manufacturing corrugated polymer pipe.